This invention relates generally to a method and apparatus for detecting the presence of harmful gases and, more specifically, to an improved gas detection apparatus which can differentiate between various noxious gases.
While gas measuring and detection instruments are well-known, recent government regulation has created a need for more precise, more reliable and faster responding instruments. The EPA (Environmental Protection Agency) has the overall responsibility for assuring that the atmosphere which we all breathe is relatively free of dangerous contaminants. In addition, the OHSA (Occupational Health and Safety Act) has set standards for the air which workers breathe to insure that the air is free from toxic, noxious and hazardous substances which might endanger the worker's health or safety.
For example, with respect to carbon monoxide, under OHSA the government has established a maximum contaminant level of 50 ppm (parts per million) average, over an eight hour time interval, and a 200 ppm maximum at any one time.
Various problems have been encountered with the prior gas detection and monitoring instruments. For example, as might be expected, the industrial needs include low cost instrumentation which is portable, accurate and reliable. Furthermore, industry needs instrumentation which can differentiate among various gases because of the different critical contaminant levels of different gases.
In addition, rather than only intermittent testing, a detector which operates continuously is important in industry so that a worker may carry the detector with him through a plant or wear it on his person to sense the presence of a potentially noxious gas and to provide an alarm. While permanently mounted fixed monitors may be placed in high-risk areas, the high-risk areas must be first identified with the use of continuously operating portable instruments. It is not practical to go through a plant initially with an instrument which must be repeatedly triggered to evaluate the atmosphere. Pockets of noxious gases may be missed easily. Thus, continuous operation is an important industrial pre-requisite.
Obviously, the gas detection instruments must be of low cost since they do not add to the profitability of the product manufactured. Of course, accuracy and reliability is also of critical importance because of the potential financial penalties which may be assessed under the OHSA for non-compliance.
Furthermore, a significant factor in compliance with OHSA standards is fast response, i.e., the ability of the gas detector to respond virtually immediately to indicate both the presence and amount of toxic gases. As a practical matter, a gas detection system which requires several hours to provide an indication of toxicity level is of dubious value to the worker who has been breathing toxic fumes for those several hours.
In addition, depending on the particular environment, the gas detection instrument must be selective, i.e., the instrument should respond to the presence of a gas such as carbon monoxide and it should be able to differentiate between that gas and other gases such as hydrocarbons.
However, all types of gas detection devices prior to the present invention had at least one drawback. The chemical types of sensor, including the colorimetric systems, are relatively slow and non-continuous. A sample of gas is introduced through a powdered reagent into a tube and a visible color change or stain occurs. The length of the stain must be physically measured to determine the concentration or contamination level. Thus, systems relying on chemical reactions are slow, non-continuous and costly. Furthermore, the tubes, once stained, are not reusable.
The optical systems rely on the infrared absorption spectra of the gas. These systems are not portable and are too expensive to install at various locations in a plant. In the absence of portability, continuous sampling and prompt indications of toxicity levels, optical systems are not feasible.
A third type of system utilized to detect the presence of toxic gas is gas chromatography in which a sample of gas is injected into an absorption column and the reaction is timed and observed. Thus, a slow response and non-continuous sampling are obvious drawbacks of gas chromatography systems.
A fourth prior art system is electrochemical gas detection wherein an oxidation-reduction reaction takes place in a fuel cell with the gas being sampled serving as one electrolyte. The fuel cell generates a current proportional to the electrolyte concentration and this type of system requires a pump and filter, reference electrodes, oxidation and reduction electrodes, feedback voltage circuitry and readout circuitry as well as the secondary electrolyte (typically sulphuric acid). This type of system, however, cannot selectively identify carbon monoxide, is quite complex and not generally satisfactory.
Because of the OHSA requirement of an average contaminant level over an eight-hour period, a portable system must operate without interruption for recharging for at least an eight-hour period. The system must operate continuously since the presence of toxic gases often occurs as a small "air pocket" and intermittent sampling of the air could result in the instrument operator missing the presence of an air pocket.